docs: def get_correlation(self, t1, t2):
          """
          Computes the correlation coefficient for the specified periods.
  
          :param float t1:
              First period of interest.
  
          :param float t2:
              Second period of interest.
  
          :return float:
              The predicted correlation coefficient.
          """
  
          if t1 not in act.periods:
              raise ValueError('t1 not a valid period')
  
          if t2 not in act.periods:
              raise ValueError('t2 not a valid period')
  
          return act.coeff_table[act.periods.index(t1)][act.periods.index(t2)]